1. Field of the Invention
This invention relates to a process for producing sodium hypochlorite. More specifically, it relates to a process for producing sodium hypochlorite which is useful for oxidizing treatment in sterilization, decolorization, deodorization, iron elimination and manganese elimination of river water, city water, sewage water, waste water, etc. and for other purposes.
2. Description of the Prior Art
Generally, sodium hypochlorite is produced by reacting a concentrated sodium hydroxide solution with chlorine gas, and commercially available grades have an available chlorine concentration of about 12% by weight. Commercially available aqueous solutions of sodium hypochloride are affected by temperature, impurities, sunlight, etc. during storage and undergo decomposition or a chemical reaction, thus causing a reduction in the concentration of available chlorine. The reduction in the available chlorine concentration is especially outstanding in the summertime, and generally about 25% of the available chlorine is lost.
In an attempt to avoid this reduction in available chlorine concentration, a method can be employed which involves providing a non-diaphragm electrolytic cell at a site adjacent a water treatment facility, and feeding an aqueous solution of sodium hypochlorite formed by electrolysis of an aqueous solution of sodium chloride directly to the adjacent treatment facility. In this case, too, there are causes which reduce the amount of available chlorine in the production of an aqueous solution of sodium hypochlorite.
The amount of available chlorine represents the amount of chlorine in the aqueous solution of sodium hypochlorite, and is generally expressed by the following equation. EQU Amount of Available Chlorine (g/l)=2.times.(chlorine in NaClO)
one cause of the reduction in the amount of available chlorine is that hypochlorite ion is electrochemically reduced at the cathode according to the following reactions. EQU HClO+H.sup.+ +2e.fwdarw.HCl+OH.sup.- ( 1) EQU clO.sup.- +2H.sup.+ +2e.fwdarw.HCl+OH.sup.- ( 2) EQU (clO.sup.- +H.sup.+ +2e.fwdarw.Cl.sup.- +OH.sup.-)
Another cause for the reduction of the amount of available chlorine is the formation of sodium chlorate in the solution by the reaction (3) shown below. EQU ClO.sup.- +2HClO.fwdarw.ClO.sub.3.sup.- +2HCl (3)
Because of these causes, the ratio of effective utilization of the sodium chloride and the current efficiency are poor in conventional non-diaphragm electrolytic cells for production of sodium hypochlorhite, and the cost of operation is very high.
Further, since the rate of reaction (3) is expressed by the equation d[ClO.sub.3.sup.- ]/dt=k]ClO.sup.- ][HClO].sup.2, the amount of chlorate ion formed increases with higher concentrations of available chlorine and higher temperatures. Hence, the current efficiency and the ratio of utilization of the starting salt both decrease even more in the non-diaphragm electrolytic cell for producing sodium hypochlorite.
U.S. Pat. No. 3,917,518 discloses a process for the production of hypochlorite solutions by electrolysis of aqueous chloride solutions. The effect of temperature on the formation of sodium chlorate appears to be disclosed but the effect of the specific ratio of the actual area of the anode to the actual area of the cathode is not disclosed not taught.
U.S. Pat. Nos. 3,849,281 and 3,819,504 are directed to an electrolytic cell for the production of alkali metal hypochlorites, specifically using a plurality of unit cells in U.S. Pat. No. 3,849,281, with particular electrode constructions. In neither of these two U.S. patents is there disclosure that the surface area of the anode to the surface area of the cathode should have a particular relationship in order to achieve improved current efficiencies.